A radial turbine is a practical device for converting gas pressure and temperature to shaft power. The majority of radial turbines incorporate fixed geometry nozzle vanes, or airfoils, to optimally guide the gas entering the rotor. In such cases the principal flow parameters, such as pressure, mass flow rate, and temperature, remain in fixed proportion, and cannot be individually controlled. The variable area nozzle provides an additional degree of control freedom, permitting independent control over inlet temperature, pressure, and flow through the turbine stage. Many examples of variable area nozzles have been deployed on such products as hover craft and turbochargers.
Most practical variable area nozzles incorporate a mechanism to rotate the nozzle vane. This is particularly critical for gas turbines where the inlet gas temperature is typically in the range from about 1000 degrees F. to 2500 degrees F. Changing the stagger angle alters the throat between nozzle vanes and changes the flow angle entering the rotor. To permit free rotation of the common variable position nozzle, clearance between the nozzle back-wall and the shroud line, or static turbine structure, is required. This clearance represents a performance loss as a percentage of the air entering the rotor will flow beneath or over top of the nozzle vane with a largely radial trajectory. This flow vector is not optimal for work extraction within the rotor, and generally contributes to inefficiency within the turbine stage.